Level scheme showing the electronic X, A and B states of N2+. (IMAGE)
Caption
Figure: Level scheme showing the electronic X, A and B states of N2+. Standard tunnelling ionization by the 800 nm pump pulse cannot induce a population inversion between the X and B states, necessary for lasing at the B ® X transition at 391 nm, shown by blue arrows. Population inversion may occur when the X state gets depopulated by rapid transfer to the A state, as shown with the red arrow. Another possibility is the simultaneous coupling of the 800 nm and 391 nm light fields coupling the three electronic states in a V-level scheme, for which lasing can occur with population inversion. Ultrafast nitrogen K-edge spectroscopy upon strong field ionization of molecular nitrogen, where the transient population of the X and B states are found to be of similar magnitude while that of the A state remains minor, strongly points to the occurrence of the V-level scheme mechanism.
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MBI / E.T.J. Nibbering
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